Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory and experiment

Knorren, R.; Bennemann, K. H.; Burgermeister, R.; Aeschlimann, Martin

doi:10.1103/physrevb.61.9427

Cited by 169 publications

(194 citation statements)

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“…4C, Inset). We note that because the unoccupied states above the Fermi energy in Ni are dominated by electronic states with minority-spin polarization (↓), spin-dependent scattering needs to be taken into account, which results in spin-dependent excitedelectron lifetime in the ferromagnetic materials such as Ni (5,6,46). Here M σσ is the spin-dependent, crystal momentum-and orbital-averaged Coulomb matrix element, assuming M ↑↑ = M ↓↓ and M ↑↓ = M ↓↑ (see S7.…”

Section: Discussionmentioning

confidence: 99%

“…Using femtosecond lasers in combination with advanced spectroscopies, it is possible to measure the lifetime of excited charges and spins directly in the time domain (1). To date, such studies have been applied to a wide variety of materials, including noble metals and semiconductors (1)(2)(3)(4), ferromagnetic metals (5)(6)(7)(8), strongly correlated materials (9) and high-T c superconductors (10,11). These studies have significantly improved our understanding of the fastest coupled interactions and relaxation mechanisms in matter.…”

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confidence: 99%

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Distinguishing attosecond electron–electron scattering and screening in transition metals

Chen

Tao

Carr

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Electron-electron interactions are the fastest processes in materials, occurring on femtosecond to attosecond timescales, depending on the electronic band structure of the material and the excitation energy. Such interactions can play a dominant role in light-induced processes such as nano-enhanced plasmonics and catalysis, light harvesting, or phase transitions. However, to date it has not been possible to experimentally distinguish fundamental electron interactions such as scattering and screening. Here, we use sequences of attosecond pulses to directly measure electronelectron interactions in different bands of different materials with both simple and complex Fermi surfaces. By extracting the time delays associated with photoemission we show that the lifetime of photoelectrons from the d band of Cu are longer by ∼100 as compared with those from the same band of Ni. We attribute this to the enhanced electron-electron scattering in the unfilled d band of Ni. Using theoretical modeling, we can extract the contributions of electron-electron scattering and screening in different bands of different materials with both simple and complex Fermi surfaces. Our results also show that screening influences high-energy photoelectrons (≈20 eV) significantly less than low-energy photoelectrons. As a result, high-energy photoelectrons can serve as a direct probe of spin-dependent electron-electron scattering by neglecting screening. This can then be applied to quantifying the contribution of electron interactions and screening to low-energy excitations near the Fermi level. The information derived here provides valuable and unique information for a host of quantum materials.attosecond science | high harmonic generation | ARPES | electron-electron interactions E xcited-state electron dynamics in materials play a critical role in light-induced phase transitions in magnetic and charge density wave materials, in superdiffusive spin flow, in catalytic processes, and in many nano-enhanced processes. However, to date exploring such dynamics is challenging both experimentally and theoretically. Using femtosecond lasers in combination with advanced spectroscopies, it is possible to measure the lifetime of excited charges and spins directly in the time domain (1). To date, such studies have been applied to a wide variety of materials, including noble metals and semiconductors (1-4), ferromagnetic metals (5-8), strongly correlated materials (9) and high-T c superconductors (10, 11). These studies have significantly improved our understanding of the fastest coupled interactions and relaxation mechanisms in matter. However, to date experimental investigations of electron dynamics have been limited to femtosecond timescale processes in materials with low charge densities (9-12) or to Fermi-liquid metals with low excitation energies (<3.0 eV above E F , where E F is the Fermi energy) (3-5), due to the visible-to-UVwavelength photon energies used in these experiments. In this region, two fundamental electron interactions-electron-electron sc...

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Distinguishing attosecond electron–electron scattering and screening in transition metals

Chen

Tao

Carr

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…Among experimental methods developed for such studies one of the most suitable is the technique of time-resolved twophoton photoemission spectroscopy (TR-2PPE), which enables lifetime measurements directly in time domain at a temporal resolution of a few femtoseconds 2 . In the last decade several TR-2PPE experiments have been performed for simple metals 3 and noble metals, 4,5,6,7,8 ferromagnetic 3d metals (Fe, Co, and Ni), 9,10 and high-T C superconductors 11 . Of the paramagnetic transition metals, the electron lifetimes have been measured only for the 5d-metal tantalum.…”

Section: Introductionmentioning

confidence: 99%

Experimental time-resolved photoemission andab initiostudy of lifetimes of excited electrons in Mo and Rh

et al. 2006

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We have studied the relaxation dynamics of optically excited electrons in molybdenum and rhodium by means of time resolved two-photon photoemission spectroscopy (TR-2PPE) and ab initio electron self-energy calculations performed within the GW and GW + T approximations. Both theoretical approaches reproduce qualitatively the experimentally observed trends and differences in the lifetimes of excited electrons in molybdenum and rhodium. For excitation energies exceeding the Fermi energy by more than 1 eV, the GW + T theory yields lifetimes in quantitative agreement with the experimental results. As one of the relevant mechanisms causing different excited state lifetime in Mo and Rh we identify the occupation of the 4d bands. An increasing occupation of the 4d bands results in an efficient decrease of the lifetime even for rather small excitation energies of a few 100 meV.

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“…We demonstrate this by developing a transport equation for the super-diffusive flow of spin-polarized electrons. A few approaches to describe the electron motion have been attempted previously [10,11]. In our theory, however, we take into account the whole process of multiple, spin-conserving electron scattering events and electron cascades created by inelastic electron scattering.…”

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confidence: 99%

Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization

Battiato

Carva²,

Oppeneer³

2010

Phys. Rev. Lett.

743

698

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We propose a semi-classical model for femtosecond-laser induced demagnetization due to spinpolarized excited electron diffusion in the super-diffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.Excitation with femtosecond laser pulses is known for more than a decade to cause an ultrafast quenching of the magnetization in metallic ferromagnets [1]. The achieved demagnetization times are typically 100-300 fs for ferromagnets such as Ni [1,2]. Hence, laser-induced demagnetization opens up new, interesting routes for magnetic recording with hitherto unprecedented speeds [3]. However, in spite of the technological importance the mechanism underlying the femtosecond demagnetization remains highly controversial. A common belief is that there should exist an ultrafast channel for the dissipation of spin angular momentum [4][5][6][7][8]. Several such mechanisms through which an excited electron can undergo a spinflip in a ferromagnetic metal are currently being debated. The main proposed mechanisms for a fast spin-flip process are a Stoner excitation, an inelastic magnon scattering, an Elliott-Yafet-type of phonon scattering [4,5], spin-flip Coulomb scattering [6], laser-induced spin-flips [7], or relativistic quantum electrodynamic processes [8]. An effect that, until recently [9], has been regarded to play only a marginal role is the spin-polarized transport of laser-excited hot electrons.In this Letter we show that spin-dependent transport of laser-excited electrons provides a considerable contribution to the ultrafast demagnetization process and can even completely explain it. We demonstrate this by developing a transport equation for the super-diffusive flow of spin-polarized electrons. A few approaches to describe the electron motion have been attempted previously [10,11]. In our theory, however, we take into account the whole process of multiple, spin-conserving electron scattering events and electron cascades created by inelastic electron scattering. Also the presence of different metallic films in the probed material is treated. We solve the developed theory numerically for ferromagnetic Ni, for which the femtosecond demagnetization is well documented [1,2,12], and show that a large demagnetization in a few hundred femtoseconds is generated.The typical geometry for a femtosecond laser experiment is depicted in Fig. 1. The intense laser beam creates excited hot electrons in the ferromagnetic film, which will start to move in a random direction. Our goal is to compute the time-dependent magnetization resulting from the super-diffusive motion in the laser spot. Due to the fact that the electronic mean-free-path (up to a few tens of nm) is much smaller tha...

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Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory and experiment

Cited by 169 publications

References 46 publications

Distinguishing attosecond electron–electron scattering and screening in transition metals

Distinguishing attosecond electron–electron scattering and screening in transition metals

Experimental time-resolved photoemission andab initiostudy of lifetimes of excited electrons in Mo and Rh

Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization

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